The present invention relates to communications systems and methods, and more particularly, to communications receivers and methods of operation thereof.
Wireless communications systems are commonly employed to provide voice and data communications to subscribers. For example, analog cellular radiotelephone systems, such as those designated AMPS, ETACS, NMT-450, and NMT-900, have long been deployed successfully throughout the world. Digital cellular radiotelephone systems, such as those conforming to the North American standard IS-54 and the European standard GSM, have been in service since the early 1990""s. More recently, a wide variety of wireless digital services broadly labeled as PCS (Personal Communications Services) have been introduced, including advanced digital cellular systems conforming to standards such as IS-136 and IS-95, lower-power systems such as DECT (Digital Enhanced Cordless Telephone) and data communications services such as CDPD (Cellular Digital Packet Data). These and other systems are described in The Mobile Communications Handbook, edited by Gibson and published by CRC Press (1996).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20. The cellular radiotelephone system 20 may include one or more radiotelephones (terminals) 22, communicating with a plurality of cells 24 served by base stations 26 and a mobile telephone switching office (MTSO) 28. Although only three cells 24 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include more than one MTSO, and may serve thousands of radiotelephones.
The cells 24 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 22 and the MTSO 28, by way of the base stations 26 serving the cells 24. Each cell 24 will have allocated to it one or more control channels and one or more traffic channels. A control channel is a channel used for transmitting cell identification, paging and other control information. Traffic channels carry the voice and data information. Through the cellular network 20, a duplex radio communication link may be effected between two mobile terminals 22 or between a mobile terminal 22 and a landline telephone user 32 through a public switched telephone network (PSTN) 34. The function of a base station 26 is to handle radio communication for a cell 24. In this capacity, a base station 26 functions as a relay station for data and voice signals.
As illustrated in FIG. 2, a satellite 42 may be employed to perform similar functions to those performed by a conventional terrestrial base station, for example, to serve areas in which population is sparsely distributed or which have rugged topography that tends to make conventional landline telephone or terrestrial cellular telephone infrastructure technically or economically impractical. A satellite radiotelephone system 40 typically includes one or more satellites 42 that serve as relays or transponders between one or more earth stations 44 and terminals 23. The satellite conveys radiotelephone communications over duplex links 46 to terminals 23 and an earth station 44. The earth station 44 may in turn be connected to a public switched telephone network 34, allowing communications between satellite radiotelephones, and communications between satellite radio telephones and conventional terrestrial cellular radiotelephones or landline telephones. The satellite radiotelephone system 40 may utilize a single antenna beam covering the entire area served by the system, or, as shown, the satellite may be designed such that it produces multiple minimally-overlapping beams 48, each serving distinct geographical coverage areas 50 in the system""s service region. The coverage areas 50 serve a similar function to the cells 24 of the terrestrial cellular system 20 of FIG. 1.
Several types of access techniques are conventionally used to provide wireless services to users of wireless systems such as those illustrated in FIGS. 1 and 2. Traditional analog cellular systems generally employ a system referred to as frequency division multiple access (FDMA) to create communications channels, wherein discrete frequency bands serve as channels over which cellular terminals communicate with cellular base stations. Typically, these bands are reused in geographically separated cells in order to increase system capacity.
Modern digital wireless systems typically utilize different multiple access techniques such as time division multiple access (TDMA) and/or code division multiple access (CDMA) to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 standard, achieve increased channel capacity by using xe2x80x9cspread spectrumxe2x80x9d techniques wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading sequence, i.e., a sequence that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread-spectrum CDMA communications systems commonly use xe2x80x9cdirect sequencexe2x80x9d (DS) spread spectrum modulation. In direct sequence modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted over a communications medium, e.g., an air interface. The spreading code typically includes a sequence of xe2x80x9cchipsxe2x80x9d occurring at a chip rate that typically is much higher than the bit rate of the data being transmitted.
In a typical CDMA system, a data stream intended for a particular user (terminal) is first direct-sequence spread according to a user-specific spreading sequence, and the resultant signal is then scrambled according to a cell-specific scrambling sequence. The spread and scrambled user data stream is then transmitted in a communications medium. Spread-spectrum signals for multiple users generally combine to form a composite signal in the communications medium.
In a typical coherent DS spread spectrum receiver, a received composite signal is despread by correlating it with a spreading sequence. The despread value is weighted by the conjugate of a channel coefficient estimate, which generally removes the phase rotation and produces a soft (confidence) value that can be used to generate an estimate of the originally transmitted symbol. When multipath propagation is present, the amplitude can vary dramatically, however. Multipath propagation can also lead to time dispersion, which can cause multiple, resolvable echoes (xe2x80x9craysxe2x80x9d) of the original transmitted signal to be received at the receiver. To deal with multipath propagation effects, a typical RAKE receiver includes respective correlators time-aligned with respective ones of the echoes, generating respective despread values for each echo. The despread values are typically combined in a weighted fashion (xe2x80x9cRAKE combinedxe2x80x9d).
Receiver performance in DS-CDMA systems is generally limited by interference among signals. Because individual channels in a DS-CDMA are typically transmitted using orthogonal spreading codes and random scrambling sequences, descrambling and despreading operations at a receiver provide a degree of interference suppression. However, dispersion and other effects may reduce the separation between signals, leading to interference among the channels. This interference may affect signal quality, which may reduce overall system capacity.
Traditional RAKE receivers typically treat interference as noise. Consequently, when interference is high, the performance of such receivers may be significantly degraded. Accordingly, interference cancellation techniques have been proposed, including multiuser interference cancellation techniques such as joint demodulation, decorrelation and successive cancellation. Unfortunately, these techniques can be highly complex and power consuming, characteristics which may be disadvantageous in power limited devices such as battery operated mobile cellular telephones.
A few single-user interference cancellation techniques have also been proposed. One approach employs adaptive despreading using a minimum mean square error (MMSE) criterion. According to such an approach, a desired signal is effectively projected away from an N-dimensional space formed by spreading sequences used by interfering other users. Another interference-cancellation technique has been proposed in U.S. Pat. No. 5,572,552 to Dent et al. This technique can be effective at canceling interference when the communications medium is dispersive, i.e., when the receiver receives a large number of echoes. However, this technique may be less effective when the medium is not highly dispersive.
In light of the foregoing, it is an object of the present invention to provide communications apparatus and methods that can provide effective interference cancellation in CDMA communications systems.
It is another object of the present invention to provide communications apparatus and methods that can provide effective interference cancellation in systems using scrambling sequences.
It is yet another object of the present invention to provide communications apparatus and methods that can provide effective interference cancellation without undue complexity and/or power consumption.
These and other objects, features and advantages may be achieved, according to the present invention, by communications apparatus and methods in which a communications signal that includes components of both a desired signal and an interfering signal, is correlated with both the desired signal""s modulation sequence and with a combination of the desired signal""s modulation sequence and a complex component of the interfering signal""s modulation sequence, for example, the interfering signal""s complex scrambling sequence, to produce respective first and second correlation outputs. The first and second correlation outputs are then combined, preferably in a weighted fashion that trades off interference cancellation and suppression of interference from signals modulated orthogonally to the desired signal, to generate an estimate of information represented by the desired signal.
In preferred wireless CDMA cellular embodiments of the present invention, knowledge of the desired signal""s scrambling sequence and spreading sequence, along with knowledge of an interferer""s complex scrambling sequence, can be used to achieve effective interference cancellation. Because the desired signal""s modulation structure is typically known and knowledge of interferer scrambling sequences can typically be straightforwardly obtained, the present invention can provide effective interference cancellation without undue complexity or cost. According to other aspects of the invention, interference cancellation can be traded off against other performance criteria, such as providing reduced interference from orthogonal-code signals transmitted from the same base station as the desired signal.
In particular, according to an embodiment of the present invention, a communications signal including components associated with first and second signals that are modulated according to respective first and second modulation sequences is processed to generate a baseband signal. The baseband signal is correlated with the first modulation sequence to produce a first correlation output. The baseband signal is also correlated with a combination of the first modulation sequence and a complex component of the second modulation sequence to generate a second correlation output. The first and second correlation outputs are combined to generate an estimate of information represented by the first signal, for example, by combining the first and second correlation outputs to produce a first RAKE finger output and combining the first RAKE finger output with a second RAKE finger output to generate an estimate of information represented by the first signal.
According to one preferred embodiment of the present invention, the first signal is modulated according to a combination of a first scrambling sequence and a first spreading sequence, and the second signal is modulated according a combination of a second scrambling sequence and a second spreading sequence. The step of correlating the baseband signal with the first modulation sequence includes the steps of descrambling the baseband signal with a complex conjugate of the first scrambling sequence to generate a first plurality of descrambled values, and correlating the first plurality of descrambled values with the first spreading sequence to generate the first correlation output. The step of correlating the baseband signal with a combination of the first modulation sequence and a complex component of the second modulation sequence includes the steps of descrambling the baseband signal with the first scrambling sequence to generate a second plurality of descrambled values, and correlating the second plurality of descrambled values with a product of the first spreading sequence, an in-phase component of the second scrambling sequence and a quadrature component of the second scrambling sequence to generate the second correlation output.
According to a zero-forcing solution aspect of the present invention, the first and second correlation outputs are combined according to channel estimates for the first and second signals. In a combined zero-forcing/conventional approach, the noise power and interference power are estimated for the first and second signals, respectively, and used, along with the channel estimates, to weightedly combine the first and second correlation outputs. Weighting factors for combining the first and second correlation outputs may also be determined using adaptive techniques, for example, by determining the weighting factors from known information, such as pilot channel or pilot symbol values, or from estimated information, such as decoded symbols having a high level of confidence.
According to other aspects of the present invention, the communications signal includes a component associated with a third signal that is modulated according to a combination of the first scrambling sequence and a third spreading sequence that is orthogonal to the first spreading sequence. According to a xe2x80x9cbalanced rotationxe2x80x9d aspect, the step of combining the first and second correlation outputs is preceded by the step of determining a minimum rotation angle that minimizes interference associated with the third signal. The first and second correlation outputs are combined according to the determined minimum rotation angle to reduce interference arising from the third signal. According to an xe2x80x9cequalizationxe2x80x9d aspect, the step of combining the first and second correlation outputs is preceded by the step of equalizing the first and second correlation outputs to control interference associated with the third signal. The equalized first and second correlation values are then combined to produce an estimate of information represented by the first signal.
According to another embodiment of the present invention, equalization may be advantageously implemented by multiplying the first plurality of descrambled values, produced by descrambling the baseband signal with the conjugate of the first scrambling sequence, by the first spreading sequence to generate scaled descrambled values. The scaled descrambled values are then sorted into first and second sets based on corresponding values of an indicator function comprising a product of in-phase and quadrature components of the first and second scrambling sequences. Respective ones of the first and second sets of scaled descrambled values are then accumulated to generate respective first and second sets of partial correlation output values. Similarly, the second plurality of descrambled values, produced by descrambling the baseband signal with the first scrambling sequence, are multiplied by a product of the first spreading sequence, an in-phase component of the second scrambling sequence and a quadrature component of the second scrambling sequence to generate scaled descrambled values. The scaled descrambled values are sorted into third and fourth sets based on corresponding values of the indicator function, and respective ones of the third and fourth sets of scaled descrambled values are accumulated to generate respective third and fourth sets of partial correlation output values. The first, second, third and fourth sets of partial correlation output values are then combined to generate a finger output.
According to another aspect of the present invention, a receiver for processing a communications signal that includes components associated with first and second signals that are modulated according to respective first and second modulation sequences includes a converter operative to produce a baseband signal from the communications signal. A complex interference orthogonalizing baseband processor is responsive to the converter and operative to correlate the baseband signal with the first modulation sequence to produce a first correlation output, to correlate the baseband signal with a combination of the first modulation sequence and a complex component of the second modulation sequence to generate a second correlation output, and to combine the first and second correlation outputs to generate an estimate of information represented by the first signal.
According to yet another embodiment of the present invention, the complex interference orthogonalizing baseband processor includes a first correlator operative to correlate the baseband signal with the first modulation sequence to produce a first correlation output. A second correlator is operative to correlate the baseband signal with a combination of the first modulation sequence and a complex component of the second modulation sequence to generate a second correlation output. A combiner is responsive to the first and second correlators and operative to combine the first and second correlation outputs to generate an estimate of information represented by the first signal.
According to another embodiment of the present invention, the first signal is modulated according to a combination of a first scrambling sequence and a first spreading sequence, and the second signal is modulated according to a combination of a second scrambling sequence and a second spreading sequence. The first correlator includes a first descrambler operative to descramble the baseband signal with a complex conjugate of the first scrambling sequence to generate a first plurality of descrambled values, and a first despreader operative to correlate the first plurality of descrambled values with the first spreading sequence to generate the first correlation output. The second correlator includes a second descrambler operative to descramble the baseband signal with the first scrambling sequence to generate a second plurality of descrambled values, and a second despreader operative to correlate the second plurality of descrambled values with a product of the first spreading sequence, an in-phase component of the second scrambling sequence and a quadrature component of the second scrambling sequence to generate the second correlation output.
Improved receiver methods and apparatus that can achieve interference cancellation without undue complexity may thereby be provided.